Researchers have discovered a new human disease in the Northeast transmitted by the same common deer tick that can infect people with Lyme disease. The bacterial illness causes flu-like symptoms, the researchers from Tufts, Yale, and other institutions reported Wednesday, but they also described the case of an 80-year-old woman who became confused and withdrawn, lost weight, and developed hearing difficulty and a wobbly gait. The woman, from New Jersey, recovered after receiving antibiotics.

Researchers estimate that 1 percent of the population in areas where Lyme disease is widespread -- such as western Massachusetts and Cape Cod and the Islands -- may be infected by the new bacteria, which can be transmitted by the tick when it is as small as a poppy seed. Lyme disease is thought to be 7 to 10 times more prevalent in these areas.

The discovery, disclosed in a paper and letter in the New England Journal of Medicine, marks the fifth human illness spread by deer ticks in the region, highlighting growing concerns about the threat posed by ticks and the burgeoning population of their hosts -- deer. The disease is so new it remains unnamed and there is no readily-available test for doctors to screen for it, although some are being developed.

It was right under our nose the whole time, said Sam Telford, a professor at Tufts Cummings School of Veterinary Medicine who studies tick-borne diseases, and one of the authors on the paper about the elderly woman. He said the bacterium, known as Borrelia miyamotoi, has been known to exist in deer ticks for about decade. But it was not believed to cause human illness until researchers last year linked it to 46 sick people in Russia, some with relapsing fevers.